Stabilizing nanograins without additives

At this point, "nano" has become somewhat of a buzz word and inside joke among materials scientists. When writing proposals, we often suggest ridiculous ways to get "nano" into the title and abstract, assuming that it is the magic passphrase for funding. Really, those of us stuck in the micro- world are just jealous of our nano-brethren because nanomaterials really do have some amazing properties and possibilities—facts that have lead to all the media coverage and growing public consciousness of nano.

For all their promise, nanomaterials have one nagging flaw—thermal stability. Whether creating nanograined solids or nanoparticles, a huge amount of energy is added to the system as surface area. The surface energy is so high that the nanograins/particles will grow into micrograins/particles at room temperature, destroying all the enhanced properties from their original nanostructure. In fact, gold nanoparticles below 6nm in diameter have been shown to melt at room temperature.

Computer simulation
of a polycrystalline microstructure.
Credit: ORNL

A variety of methods have been used to stabilize nanograined metals to moderate temperatures (a few hundred degrees celsius), but they almost all rely on additives that reduce the mechanical properties over the pure system. A recent article in Applied Physics Letters reports a method for stabilizing nanograins in copper without using such additives.

To make the copper samples, the researchers used severe plastic deformation, by far the most common method for producing nanograined metals, but they noted and exploited a seemingly minor detail; certain strain rates and temperatures lead to an unusually high population of grains that were highly oriented. That is, a grain's nearest neighbors were likely to have an almost identical orientation. Normally this would not be any cause for celebration, but recent data has shown that triple junctions between highly oriented grains have extremely low mobility.

A triple junction is a place in a microstructure where three grains come together and they have little effect on most microstructures because they make up a tiny fraction of the total volume of the system. In nanograined materials, triple junctions become increasingly important because they occupy substantially larger volume fractions as the grain size decreases. In these highly oriented systems where the triple junctions are particularly slow moving, grain growth is prevented because these junctions effectively pin the grain boundaries to their current location. Oriented samples showed almost no decrease in mechanical properties or grain size after 1500 hours at room temperature while conventional nanograin copper samples returned to their micrograined properties over the same time span.

Unfortunately, the article lacks any higher temperature data which suggests that the those results may not be as convincing, but this proof of concept is a good starting point for future studies in more interesting systems.